The present invention relates generally to a non-polar ultraviolet light-emitting device and method of manufacturing a light-emitting device.
Group III nitride compound semiconductors such as, for instance, gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN) (hereinafter also referred to as a “Group III-nitride semiconductor” or “III-nitrides”) have been gaining attention as a material for semiconductor devices that emit green, blue or ultraviolet light. A light-emitting device, which together with the acronym LED when used herein, will for convenience also refer to both a light-emitting diode and laser diode (LD) unless otherwise specified. Of particular interest herein are LED's which emit in the ultraviolet portion of the electromagnetic spectrum. An LED that emits ultraviolet radiation is expected to find applications in the field of ultraviolet curing, phototherapy, water and air purification, bio-detection, and germicidal treatment. The ultraviolet portion of the electromagnetic spectrum is typically radiation with a wavelength of 200-400 nm and radiation of less than about 300 is often referred to in the art as deep-UV. For the purposes of discussion the ultraviolet portion of the electromagnetic spectrum is often further subdivided by wavelength into UVA (315-380 nm), UVB (280-315 nm) and UVC (<280 nm). For the purposes of the present invention deep-UV (or DUV) refers to wavelengths of 200-300 nm and ultraviolet (or UV) refers to wavelengths of 200-400 nm.
UV emitting LED's are difficult to manufacture for a number of reasons. For example, defects arise from lattice and thermal mismatch between the group III-Nitride based semiconductor layers and the substrate. In addition, impurities and tilt boundaries result in the formation of crystalline defects. These defects have been shown to reduce the efficiency and lifetime of LEDs and LDs fabricated from these materials.
One way to reduce the dislocation density is based on the use of epitaxial lateral overgrowth (ELOG), which is a well-known technique in the prior art. With this method, the dislocation density can be reduced to about 105 cm−2 to 106 cm−2. This method, however, has been shown to be ineffective for the growth of aluminum-containing III-Nitride based semiconductors because of the tendency for the aluminum to stick to the masked material and disrupt the lateral overgrowth. Several variations of this approach have also been demonstrated including PENDEO epitaxy, and FACELO growth. All of these techniques suffer from the same limitation as the ELOG approach for aluminum containing III-Nitride materials.
Additionally, a technique called cantilever epitaxy involves growth from pillars that are defined through etching as opposed to, for example, masking.
Several other approaches to dislocation reduction have been reported that do not involve selective area growth including inserting an interlayer between the substrate and the semiconductor layer to relieve strain, filtering dislocations by bending them into each other by controlling surface facet formation or by inserting a Group III-Nitride super-lattice layer as described in Applied Physics Letters, Jul. 22, 2002; Volume 81, Issue 4, pp. 604-606, between the buffer layer and the active layer.
Accordingly, several research groups at present are developing III-nitride deep ultraviolet light emitting diodes for applications in air and water purification and bio-medical systems. Milli-watt power DUV LEDs on sapphire substrates with AlGaN multiple quantum well (MQW) active regions have been previously reported for the UVA, UVB and the UVC regions. The LED design used in the prior art benefited from several key innovations, namely: (1) the use of pulsed atomic layer epitaxy (PALE) to improve the quality of the buffer AlN layer; (2) the use of a PALE deposited AlN/AlxGa1-xN, short-period super-lattice layer insertion between the buffer AlN and the n-contact AlGaN layer for controlling the thin-film stress and mitigating epilayer cracking; and (3) a p-GaN/p-AlGaN hetero-junction contact layer for improved hole injection.
In preparing semiconductor LED's of AlxInyGa1-x-yN wherein 0≦x≦1, 0≦y≦1 and 0≦x+y≦1, an AlN or GaN buffer layer is typically grown on a c-plane (0001) of a substrate and is therefore referred to as a c-plane buffer. The structure of the c-plane comprises a high density of threading dislocations which significantly reduces the lifetime of the light emitters and the manufacturing yield. In addition, the III-nitride LED's and LD's grown on the c-plane typically exhibit polarization related electric fields resulting in a quantum confined Stark effect. These electrostatic fields separate the electron and hole envelope wave functions in a heterostructure such as a quantum well. The consequent reduction in the envelope wave-function overlap results in a lower radiative efficiency for light-emitting devices. For LED's emitting in the visible portion of the electromagnetic spectrum the problems have been mitigated by growing on a non-polar GaN substrate using standard deposition techniques such as MOCVD or MBE. While helpful, this problem has not proven satisfactory for UV LED's which require high aluminum containing AlInGaN quantum wells when conventional deposition techniques are used. Under standard deposition conditions the epilayer quality suffers due to high gas phase reaction of the precursor sources. In addition, high aluminum containing AlInGaN device structure cracks due to tensile stress when grown directly on GaN substrates.
There still remains a need for higher quality, more reliable, more robust, deep UV light-emitting diodes and laser diodes and a method for preparing them.